In fossil-fired power plants for generating electrical energy, the combustion of a fossil fuel gives rise to a flue gas containing carbon dioxide. To avoid or reduce emissions of carbon dioxide, carbon dioxide has to be separated from the flue gases. To separate carbon dioxide from a gas mixture, various methods are known in general. The method of absorption/desorption is customary particularly for separating carbon dioxide from a flue gas after a combustion process. In this case, on an industrial scale, carbon dioxide is washed out of the flue gas by means of an absorbent.
In a conventional absorption/desorption process, the flue gas is brought into contact in an absorption column with a selective absorbent as washing agent and is in this case absorbed by the washing agent. The absorbent, then laden with carbon dioxide, is conducted into a desorption column in order to separate the carbon dioxide and regenerate the absorbent. The laden absorbent is heated, carbon dioxide being desorbed again from the absorbent and a regenerated absorbent being formed. The regenerated absorbent is conducted once again to the absorber column where it can take up carbon dioxide again from the exhaust gas containing carbon dioxide.
Customary absorbents exhibit good selectivity and a high capacity for the carbon dioxide to be separated. Absorbents are especially suitable which are based on amines, such as, for example, monoethanolamine. In the chemical industry, too, amine solutions are usually employed as absorbents.
By the absorbent being in contact with the flue gas, a large quantity of contaminants from the flue gas and flue gas bi-products are introduced, in addition to the carbon dioxide, into the absorbent. Also due to constant thermal load, in the course of time the absorbent is damaged in an absorption/desorption process. The absorbent consequently has to be replaced continuously. In this case, a comparatively large quantity of unused absorbent, too, is always extracted from the absorption/desorption process together with the contaminants and degradation products.
When amine-based absorbents are used, the amines can be recovered by distillation. Amine solutions form, with the acid flue gas secondary components, stable salts. Owing to the distillative purification of the amine solution, that is to say owing to the evaporation of the more easily volatile amines and their subsequent condensation, it is possible to separate the high-boiling contaminants and therefore to purify the amine solution. However, the appreciable vapor pressure of the amines, which is utilized for distillative purification, also means that, during the absorption/desorption process, a small fraction of amines is discharged together with the purified flue gas into the environment, thus leading to undesirable air pollution. Moreover, the distillative purification methods necessitate a high outlay in energy terms.
By contrast, amino acid salts have no measurable vapor pressure and are therefore also not discharged together with the flue gas into the environment. However, for this reason, it is also not possible to carry out distillative processing of an amino acid salt solution. There has been no purification method known hitherto for an amino acid salt solution. The extracted quantity of used amino acid salt solution therefore has to be disposed of completely.